Controls on Greenhouse Gas Emissions and Redox-Sensitive Processes in Terrestrial Ecosystems
We focus on understanding the controls on soil emissions of methane (CH4) and nitrous oxide (N2O), both potent greenhouse gases that can be simultaneously produced and consumed in soil through redox sensitive processes. Our understanding of the controls on soil-atmosphere exchange of these gases is limited by measuring net fluxes because the mechanisms driving the production and consumption processes are different. We use the stable isotope trace gas pool dilution technique to measure simultaneous gross production and consumption of CH4 and N2O in the field. In a salt marsh, we found that N2O dynamics did not follow a redox gradient across the landscape. Instead, net N2O fluxes were lowest in the mid marsh where the soils were consistently a net sink for atmospheric N2O. The patterns in net N2O fluxes were driven by differences in gross N2O production among marsh zones because gross N2O consumption rates were similar across the landscape. In a drained peatland, we surprisingly found that soil from below the water table had the highest CH4 production potential under anoxic conditions and also the highest CH4 consumption potential under oxic conditions, suggesting that CH4 dynamics in the deep soil drive soil CH4 emissions. In an active corn field, we observed that gross production and consumption of both CH4 and N2O increased over the course of the growing season, possibly reflecting the accumulation of plant carbon inputs to the soil. These three studies together demonstrate that measurements of gross fluxes of N2O and CH4 can provide additional insight into the controls on soil N2O and CH4 emissions across a range of terrestrial ecosystems. We are now using the stable isotope trace gas pool dilution technique to explore mechanistic controls on CH4 and N2O emissions from intensively managed landscapes in the Midwest.
We are also exploring the environmental and genetic potential for dissimilatory nitrate reduction to ammonium (DNRA), a microbially mediated process previously thought to be restricted to highly reducing environments such as animal guts and wetland sediments, to contribute to nitrate retention in Midwest agricultural systems. In collaboration with Dr. Angela Kent, a microbial ecologist in the UIUC Department of Natural Resources and Environmental Sciences, we are currently surveying the environmental and genetic potential for DNRA in many existing agricultural field trials that provide contrasting soil conditions (e.g., annual corn and soybean crops versus perennial bioenergy crops, till versus no-till, fertilized versus unfertilized, etc.). We are measuring DNRA rates using stable isotope approaches, as well as the abundance of nrfA (the functional gene responsible for the key step in DNRA) and the microbial community composition using high throughput qPCR and Illumina sequencing based on the Fluidigm multiplexed microfluidics platform. This work is funded by the USDA and the Illinois Nutrient Retention and Education Council.
The Effects of Plant Community Composition on Ecosystem Nitrogen and Carbon Dynamics
Invasive species can alter the structure and function of ecosystems, potentially creating positive feedbacks to invasion by changing ecosystem nitrogen (N) cycling. We explored how invasion by Lepidium latifolium, perennial pepperweed, affected soil N cycling in a grassland dominated by Hordeum murinum, an invasive annual grass. We found that soil N pools and gross N cycling were lower under pepperweed at peak growth, but after senescence, gross N mineralization rates were higher under pepperweed. In collaboration with Dr. Tony Yannarell, a microbial ecologist in the UIUC Department of Natural Resources and Environmental Sciences, we are now interested in exploring the mechanisms through which garlic mustard, an invasive species prevalent in the Midwest, can alter soil nitrogen cycling and how those mechanisms may differ with plant phenology.
We are also exploring the effect of plant community composition on ecosystem service production in Midwest agricultural systems. The Midwest is currently dominated by annual monocultures (corn-soybean rotations) which require high inputs of fertilizers, pesticides, and energy, resulting in a plethora of negative environmental impacts. A transformative approach to sustainable agriculture in the Midwest uses woody polycultures–which consist of at least one fruit- or nut-producing woody crop grown with another perennial crop–to produce healthy foods, a diversified income stream for farmers, and enhanced ecosystem services such as ecosystem nutrient retention and carbon sequestration. We are part of an interdisciplinary team funded by the UIUC Institute for Sustainability, Energy, and Environment to study woody polycultures (also known as production agroforestry) on a new 30-acre research farm planted in Urbana, IL in 2015 and on working farms throughout the Midwest.
Coupling of Biogeochemical Cycling Beyond Carbon and Nitrogen
Soils are generally rich in iron, the fourth most abundant element in Earth’s crust. Changes in the redox state of iron can be coupled to the biogeochemical cycling of carbon, nitrogen, and phosphorus through both biotic and abiotic processes. The importance of iron in catalyzing redox-driven biogeochemical cycling has been underappreciated in terrestrial ecosystems because they are not typically thought of as anaerobic environments. However, soils can experience anaerobic conditions following rainfall events or in microsites of high biological oxygen consumption. Our group focuses on a new pathway for nitrogen loss from terrestrial ecosystems, iron reduction coupled to anaerobic ammonium oxidation (termed Feammox). This pathway can lead to the production of nitrite or nitrate, which can contribute to nitrous oxide (N2O) emissions if denitrified. However, it can also lead directly to dinitrogen (N2) production from ammonium, thus short circuiting the soil nitrogen cycle and bypassing the potential for N2O production. We are currently exploring the controls on Feammox in a humid tropical forest soil and a temperate grassland soil.